CNT-infused carbon fiber materials and process therefor

ABSTRACT

A composition includes a carbon nanotube (CNT)-infused carbon fiber material that includes a carbon fiber material of spoolable dimensions and carbon nanotubes (CNTs) infused to the carbon fiber material. The infused CNTs are uniform in length and uniform in distribution. The CNT infused carbon fiber material also includes a barrier coating conformally disposed about the carbon fiber material, while the CNTs are substantially free of the barrier coating. A continuous CNT infusion process includes: (a) functionalizing a carbon fiber material; (b) disposing a barrier coating on the functionalized carbon fiber material (c) disposing a carbon nanotube (CNT)-forming catalyst on the functionalized carbon fiber material; and (d) synthesizing carbon nanotubes, thereby forming a carbon nanotube-infused carbon fiber material.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a divisional of U.S. patent application Ser. No.12/611,101, filed Nov. 2, 2009, which is a continuation-in-part of U.S.Pat. No. 8,158,217, filed on Jan. 3, 2007 and issued on Apr. 17, 2012.This application claims the benefit of priority under 35 U.S.C. §119 toU.S. Provisional Application Nos. 61/168,516, filed Apr. 10, 2009;61/169,055 filed Apr. 14, 2009; 61/155,935, filed Feb. 27, 2009;61/157,096, filed Mar. 3, 2009; and 61/182,153, filed May 29, 2009, allof which are incorporated herein by reference in their entirety.

FIELD OF THE INVENTION

The present invention relates to fiber materials, more specifically tocarbon fiber materials modified with carbon nanotubes.

BACKGROUND OF THE INVENTION

Fiber materials are used for many different applications in a widevariety of industries, such as the commercial aviation, recreation,industrial and transportation industries. Commonly-used fiber materialsfor these and other applications include carbon fiber, cellulosic fiber,glass fiber, metal fiber, ceramic fiber and aramid fiber, for example.

Carbon fiber is routinely manufactured with sizing agents to protect thematerial from environmental degradation. Additionally, other physicalstresses can compromise carbon fiber integrity such as compressiveforces and self abrasion. Many sizing formulations used to protectcarbon fibers against these vulnerabilities are proprietary in natureand are designed to interface with specific resin types. To realize thebenefit of carbon fiber material properties in a composite, there mustbe a good interface between the carbon fibers and the matrix. The sizingemployed on a carbon fiber can provide a physico-chemical link betweenfiber and the resin matrix and thus affects the mechanical and chemicalproperties of the composite.

However, most conventional sizing agents have a lower interfacialstrength than the carbon fiber material to which they are applied. As aconsequence, the strength of the sizing and its ability to withstandinterfacial stress ultimately determines the strength of the overallcomposite. Thus, using conventional sizing, the resulting composite willgenerally have a strength less than that of the carbon fiber material.

It would be useful to develop sizing agents and processes of coating thesame on carbon fiber materials to address some of the issues describedabove as well as to impart desirable characteristics to the carbon fibermaterials. The present invention satisfies this need and providesrelated advantages as well.

SUMMARY OF THE INVENTION

In some aspects, embodiments disclosed here relate to a composition thatincludes a carbon nanotube (CNT)-infused carbon fiber material. TheCNT-infused carbon fiber material includes a carbon fiber material ofspoolable dimensions and carbon nanotubes (CNTs) infused to the carbonfiber material. The infused CNTs are uniform in length and uniform indistribution. The CNT-infused carbon fiber material also includes abarrier coating conformally disposed about the carbon fiber material,while the CNTs are substantially free of the barrier coating.

In some aspects, embodiments disclosed herein related to a continuousCNT infusion process that includes: (a) functionalizing a carbon fibermaterial; (b) disposing a barrier coating on the functionalized carbonfiber material (c) disposing a carbon nanotube (CNT)-forming catalyst onthe functionalized carbon fiber material; and (d) synthesizing carbonnanotubes, thereby forming a carbon nanotube-infused carbon fibermaterial.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 shows a transmission electron microscope (TEM) image of amulti-walled CNT (MWNT) grown on AS4 carbon fiber via a continuous CVDprocess.

FIG. 2 shows a TEM image of a double-walled CNT (DWNT) grown on AS4carbon fiber via a continuous CVD process.

FIG. 3 shows a scanning electron microscope (SEM) image of CNTs growingfrom within the barrier coating where the CNT-forming nanoparticlecatalyst was mechanically infused to the carbon fiber material surface.

FIG. 4 shows a SEM image demonstrating the consistency in lengthdistribution of CNTs grown on a carbon fiber material to within 20% of atargeted length of about 40 microns.

FIG. 5 shows an SEM image demonstrating the effect of a barrier coatingon CNT growth. Dense, well aligned CNTs grew where barrier coating wasapplied and no CNTs grew where barrier coating was absent.

FIG. 6 shows a low magnification SEM of CNTs on carbon fiberdemonstrating the uniformity of CNT density across the fibers withinabout 10%.

FIG. 7 shows a process for producing CNT-infused carbon fiber materialin accordance with the illustrative embodiment of the present invention.

FIG. 8 shows how a carbon fiber material can be infused with CNTs in acontinuous process to target thermal and electrical conductivityimprovements.

FIG. 9 shows how carbon fiber material can be infused with CNTs in acontinuous process using a “reverse” barrier coating process to targetimprovements in mechanical properties, especially interfacialcharacteristics such as shear strength.

FIG. 10 shows how carbon fiber material can be infused with CNTs inanother continuous process using a “hybrid” barrier coating to targetimprovements in mechanical properties, especially interfacialcharacteristics such as shear strength and interlaminar fracturetoughness.

FIG. 11 shows the effect of infused CNTs on IM7 carbon fiber oninterlaminar fracture toughness. The baseline material is an unsized IM7carbon fiber, while the CNT-Infused material is an unsized carbon fiberwith 15 micron long CNTs infused on the fiber surface.

DETAILED DESCRIPTION

The present disclosure is directed, in part, to carbon nanotube-infused(“CNT-infused”) carbon fiber materials. The infusion of CNTs to thecarbon fiber material can serve many functions including, for example,as a sizing agent to protect against damage from moisture, oxidation,abrasion, and compression. A CNT-based sizing can also serve as aninterface between the carbon fiber material and a matrix material in acomposite. The CNTs can also serve as one of several sizing agentscoating the carbon fiber material.

Moreover, CNTs infused on a carbon fiber material can alter variousproperties of the carbon fiber material, such as thermal and/orelectrical conductivity, and/or tensile strength, for example. Theprocesses employed to make CNT-infused carbon fiber materials provideCNTs with substantially uniform length and distribution to impart theiruseful properties uniformly over the carbon fiber material that is beingmodified. Furthermore, the processes disclosed herein are suitable forthe generation of CNT-infused carbon fiber materials of spoolabledimensions.

The present disclosure is also directed, in part, to processes formaking CNT-infused carbon fiber materials. The processes disclosedherein can be applied to nascent carbon fiber materials generated denovo before, or in lieu of, application of a typical sizing solution tothe carbon fiber material. Alternatively, the processes disclosed hereincan utilize a commercial carbon fiber material, for example, a carbontow, that already has a sizing applied to its surface. In suchembodiments, the sizing can be removed to provide a direct interfacebetween the carbon fiber material and the synthesized CNTs, although abarrier coating and/or transition metal particle can serve as anintermediate layer providing indirect infusion, as explained furtherbelow. After CNT synthesis further sizing agents can be applied to thecarbon fiber material as desired.

The processes described herein allow for the continuous production ofcarbon nanotubes of uniform length and distribution along spoolablelengths of tow, tapes, fabrics and other 3D woven structures. Whilevarious mats, woven and non-woven fabrics and the like can befunctionalized by processes of the invention, it is also possible togenerate such higher ordered structures from the parent tow, yarn or thelike after CNT functionalization of these parent materials. For example,a CNT-infused woven fabric can be generated from a CNT-infused carbonfiber tow.

As used herein the term “carbon fiber material” refers to any materialwhich has carbon fiber as its elementary structural component. The termencompasses fibers, filaments, yarns, tows, tows, tapes, woven andnon-woven fabrics, plies, mats, and the like.

As used herein the term “spoolable dimensions” refers to carbon fibermaterials having at least one dimension that is not limited in length,allowing for the material to be stored on a spool or mandrel. Carbonfiber materials of “spoolable dimensions” have at least one dimensionthat indicates the use of either batch or continuous processing for CNTinfusion as described herein. One carbon fiber material of spoolabledimensions that is commercially available is exemplified by AS4 12 kcarbon fiber tow with a tex value of 800 (1 tex=1 g/1,000 m) or 620yard/lb (Grafil, Inc., Sacramento, Calif.). Commercial carbon fiber tow,in particular, can be obtained in 5, 10, 20, 50, and 100 lb. (for spoolshaving high weight, usually a 3 k/12K tow) spools, for example, althoughlarger spools may require special order. Processes of the inventionoperate readily with 5 to 20 lb. spools, although larger spools areusable. Moreover, a pre-process operation can be incorporated thatdivides very large spoolable lengths, for example 100 lb. or more, intoeasy to handle dimensions, such as two 50 lb spools.

As used herein, the term “carbon nanotube” (CNT, plural CNTs) refers toany of a number of cylindrically-shaped allotropes of carbon of thefullerene family including single-walled carbon nanotubes (SWNTs),double-walled carbon nanotubes (DWNTs), multi-walled carbon nanotubes(MWNTs). CNTs can be capped by a fullerene-like structure or open-ended.CNTs include those that encapsulate other materials.

As used herein “uniform in length” refers to length of CNTs grown in areactor. “Uniform length” means that the CNTs have lengths withtolerances of plus or minus about 20% of the total CNT length or less,for CNT lengths varying from between about 1 micron to about 500microns. At very short lengths, such as 1-4 microns, this error may bein a range from between about plus or minus 20% of the total CNT lengthup to about plus or minus 1 micron, that is, somewhat more than about20% of the total CNT length.

As used herein “uniform in distribution” refers to the consistency ofdensity of CNTs on a carbon fiber material. “Uniform distribution” meansthat the CNTs have a density on the carbon fiber material withtolerances of plus or minus about 10% coverage defined as the percentageof the surface area of the fiber covered by CNTs. This is equivalent to±1500 CNTs/μm² for an 8 nm diameter CNT with 5 walls. Such a figureassumes the space inside the CNTs as fillable.

As used herein, the term “infused” means bonded and “infusion” means theprocess of bonding. Such bonding can involve direct covalent bonding,ionic bonding, pi-pi, and/or van der Waals force-mediated physisorption.For example, in some embodiments, the CNTs can be directly bonded to thecarbon fiber material. Bonding can be indirect, such as the CNT infusionto the carbon fiber material via a barrier coating and/or an interveningtransition metal nanoparticle disposed between the CNTs and carbon fibermaterial. In the CNT-infused carbon fiber materials disclosed herein,the carbon nanotubes can be “infused” to the carbon fiber materialdirectly or indirectly as described above. The particular manner inwhich a CNT is “infused” to a carbon fiber materials is referred to as a“bonding motif.”

As used herein, the term “transition metal” refers to any element oralloy of elements in the d-block of the periodic table. The term“transition metal” also includes salt forms of the base transition metalelement such as oxides, carbides, nitrides, and the like.

As used herein, the term “nanoparticle” or NP (plural NPs), orgrammatical equivalents thereof refers to particles sized between about0.1 to about 100 nanometers in equivalent spherical diameter, althoughthe NPs need not be spherical in shape. Transition metal NPs, inparticular, serve as catalysts for CNT growth on the carbon fibermaterials.

As used herein, the term “sizing agent,” “fiber sizing agent,” or just“sizing,” refers collectively to materials used in the manufacture ofcarbon fibers as a coating to protect the integrity of carbon fibers,provide enhanced interfacial interactions between a carbon fiber and amatrix material in a composite, and/or alter and/or enhance particularphysical properties of a carbon fiber. In some embodiments, CNTs infusedto carbon fiber materials behave as a sizing agent.

As used herein, the term “matrix material” refers to a bulk materialthan can serve to organize sized CNT-infused carbon fiber materials inparticular orientations, including random orientation. The matrixmaterial can benefit from the presence of the CNT-infused carbon fibermaterial by imparting some aspects of the physical and/or chemicalproperties of the CNT-infused carbon fiber material to the matrixmaterial.

As used herein, the term “material residence time” refers to the amountof time a discrete point along a glass fiber material of spoolabledimensions is exposed to CNT growth conditions during the CNT infusionprocesses described herein. This definition includes the residence timewhen employing multiple CNT growth chambers.

As used herein, the term “linespeed” refers to the speed at which aglass fiber material of spoolable dimensions can be fed through the CNTinfusion processes described herein, where linespeed is a velocitydetermined by dividing CNT chamber(s) length by the material residencetime.

In some embodiments, the present invention provides a composition thatincludes a carbon nanotube (CNT)-infused carbon fiber material. TheCNT-infused carbon fiber material includes a carbon fiber material ofspoolable dimensions, a barrier coating conformally disposed about thecarbon fiber material, and carbon nanotubes (CNTs) infused to the carbonfiber material. The infusion of CNTs to the carbon fiber material caninclude a bonding motif of direct bonding of individual CNTs to thecarbon fiber material or indirect bonding via a transition metal NP,barrier coating, or both.

Without being bound by theory, transition metal NPs, which serve as aCNT-forming catalyst, can catalyze CNT growth by forming a CNT growthseed structure. In one embodiment, the CNT-forming catalyst can remainat the base of the carbon fiber material, locked by the barrier coating,and infused to the surface of the carbon fiber material. In such a case,the seed structure initially formed by the transition metal nanoparticlecatalyst is sufficient for continued non-catalyzed seeded CNT growthwithout allowing the catalyst to move along the leading edge of CNTgrowth, as often observed in the art. In such a case, the NP serves as apoint of attachment for the CNT to the carbon fiber material. Thepresence of the barrier coating can also lead to further indirectbonding motifs. For example, the CNT forming catalyst can be locked intothe barrier coating, as described above, but not in surface contact withcarbon fiber material. In such a case a stacked structure with thebarrier coating disposed between the CNT forming catalyst and carbonfiber material results. In either case, the CNTs formed are infused tothe carbon fiber material. In some embodiments, some barrier coatingswill still allow the CNT growth catalyst to follow the leading edge ofthe growing nanotube. In such cases, this can result in direct bondingof the CNTs to the carbon fiber material or, optionally, to the barriercoating. Regardless of the nature of the actual bonding motif formedbetween the carbon nanotubes and the carbon fiber material, the infusedCNT is robust and allows the CNT-infused carbon fiber material toexhibit carbon nanotube properties and/or characteristics.

Again, without being bound by theory, when growing CNTs on carbon fibermaterials, the elevated temperatures and/or any residual oxygen and/ormoisture that can be present in the reaction chamber can damage thecarbon fiber material. Moreover, the carbon fiber material itself can bedamaged by reaction with the CNT-forming catalyst itself. That is thecarbon fiber material can behave as a carbon feedstock to the catalystat the reaction temperatures employed for CNT synthesis. Such excesscarbon can disturb the controlled introduction of the carbon feedstockgas and can even serve to poison the catalyst by overloading it withcarbon. The barrier coating employed in the invention is designed tofacilitate CNT synthesis on carbon fiber materials. Without being boundby theory, the coating can provide a thermal barrier to heat degradationand/or can be a physical barrier preventing exposure of the carbon fibermaterial to the environment at the elevated temperatures. Alternativelyor additionally, it can minimize the surface area contact between theCNT-forming catalyst and the carbon fiber material and/or it canmitigate the exposure of the carbon fiber material to the CNT-formingcatalyst at CNT growth temperatures.

Compositions having CNT-infused carbon fiber materials are provided inwhich the CNTs are substantially uniform in length. In the continuousprocess described herein, the residence time of the carbon fibermaterial in a CNT growth chamber can be modulated to control CNT growthand ultimately, CNT length. This provides a means to control specificproperties of the CNTs grown. CNT length can also be controlled throughmodulation of the carbon feedstock and carrier gas flow rates andreaction temperature. Additional control of the CNT properties can beobtained by controlling, for example, the size of the catalyst used toprepare the CNTs. For example, 1 nm transition metal nanoparticlecatalysts can be used to provide SWNTs in particular. Larger catalystscan be used to prepare predominantly MWNTs.

Additionally, the CNT growth processes employed are useful for providinga CNT-infused carbon fiber material with uniformly distributed CNTs oncarbon fiber materials while avoiding bundling and/or aggregation of theCNTs that can occur in processes in which pre-formed CNTs are suspendedor dispersed in a solvent solution and applied by hand to the carbonfiber material. Such aggregated CNTs tend to adhere weakly to a carbonfiber material and the characteristic CNT properties are weaklyexpressed, if at all. In some embodiments, the maximum distributiondensity, expressed as percent coverage, that is, the surface area offiber covered, can be as high as about 55% assuming about 8 nm diameterCNTs with 5 walls. This coverage is calculated by considering the spaceinside the CNTs as being “fillable” space. Various distribution/densityvalues can be achieved by varying catalyst dispersion on the surface aswell as controlling gas composition and process speed. Typically for agiven set of parameters, a percent coverage within about 10% can beachieved across a fiber surface. Higher density and shorter CNTs areuseful for improving mechanical properties, while longer CNTs with lowerdensity are useful for improving thermal and electrical properties,although increased density is still favorable. A lower density canresult when longer CNTs are grown. This can be the result of the highertemperatures and more rapid growth causing lower catalyst particleyields.

The compositions of the invention having CNT-infused carbon fibermaterials can include a carbon fiber material such as a carbon filament,a carbon fiber yarn, a carbon fiber tow, a carbon tape, a carbonfiber-braid, a woven carbon fabric, a non-woven carbon fiber mat, acarbon fiber ply, and other 3D woven structures. Carbon filamentsinclude high aspect ratio carbon fibers having diameters ranging in sizefrom between about 1 micron to about 100 microns. Carbon fiber tows aregenerally compactly associated bundles of filaments and are usuallytwisted together to give yarns.

Yarns include closely associated bundles of twisted filaments. Eachfilament diameter in a yarn is relatively uniform. Yarns have varyingweights described by their ‘tex,’ expressed as weight in grams of 1000linear meters, or denier, expressed as weight in pounds of 10,000 yards,with a typical tex range usually being between about 200 tex to about2000 tex.

Tows include loosely associated bundles of untwisted filaments. As inyarns, filament diameter in a tow is generally uniform. Tows also havevarying weights and the tex range is usually between 200 tex and 2000tex. They are frequently characterized by the number of thousands offilaments in the tow, for example 12K tow, 24K tow, 48K tow, and thelike.

Carbon tapes are materials that can be assembled as weaves or canrepresent non-woven flattened tows. Carbon tapes can vary in width andare generally two-sided structures similar to ribbon. Processes of thepresent invention are compatible with CNT infusion on one or both sidesof a tape. CNT-infused tapes can resemble a “carpet” or “forest” on aflat substrate surface. Again, processes of the invention can beperformed in a continuous mode to functionalize spools of tape.

Carbon fiber-braids represent rope-like structures of densely packedcarbon fibers. Such structures can be assembled from carbon yarns, forexample. Braided structures can include a hollow portion or a braidedstructure can be assembled about another core material.

In some embodiments a number of primary carbon fiber material structurescan be organized into fabric or sheet-like structures. These include,for example, woven carbon fabrics, non-woven carbon fiber mat and carbonfiber ply, in addition to the tapes described above. Such higher orderedstructures can be assembled from parent tows, yarns, filaments or thelike, with CNTs already infused in the parent fiber. Alternatively suchstructures can serve as the substrate for the CNT infusion processesdescribed herein.

There are three types of carbon fiber which are categorized based on theprecursors used to generate the fibers, any of which can be used in theinvention: Rayon, Polyacrylonitrile (PAN) and Pitch. Carbon fiber fromrayon precursors, which are cellulosic materials, has relatively lowcarbon content at about 20% and the fibers tend to have low strength andstiffness. Polyacrylonitrile (PAN) precursors provide a carbon fiberwith a carbon content of about 55%. Carbon fiber based on a PANprecursor generally has a higher tensile strength than carbon fiberbased on other carbon fiber precursors due to a minimum of surfacedefects.

Pitch precursors based on petroleum asphalt, coal tar, and polyvinylchloride can also be used to produce carbon fiber. Although pitches arerelatively low in cost and high in carbon yield, there can be issues ofnon-uniformity in a given batch.

CNTs useful for infusion to carbon fiber materials include single-walledCNTs, double-walled CNTs, multi-walled CNTs, and mixtures thereof. Theexact CNTs to be used depends on the application of the CNT-infusedcarbon fiber. CNTs can be used for thermal and/or electricalconductivity applications, or as insulators. In some embodiments, theinfused carbon nanotubes are single-wall nanotubes. In some embodiments,the infused carbon nanotubes are multi-wall nanotubes. In someembodiments, the infused carbon nanotubes are a combination ofsingle-wall and multi-wall nanotubes. There are some differences in thecharacteristic properties of single-wall and multi-wall nanotubes that,for some end uses of the fiber, dictate the synthesis of one or theother type of nanotube. For example, single-walled nanotubes can besemi-conducting or metallic, while multi-walled nanotubes are metallic.

CNTs lend their characteristic properties such as mechanical strength,low to moderate electrical resistivity, high thermal conductivity, andthe like to the CNT-infused carbon fiber material. For example, in someembodiments, the electrical resistivity of a carbon nanotube-infusedcarbon fiber material is lower than the electrical resistivity of aparent carbon fiber material. More generally, the extent to which theresulting CNT-infused fiber expresses these characteristics can be afunction of the extent and density of coverage of the carbon fiber bythe carbon nanotubes. Any amount of the fiber surface area, from 0-55%of the fiber can be covered assuming an 8 nm diameter, 5-walled MWNT(again this calculation counts the space inside the CNTs as fillable).This number is lower for smaller diameter CNTs and more for greaterdiameter CNTs. 55% surface area coverage is equivalent to about 15,000CNTs/micron². Further CNT properties can be imparted to the carbon fibermaterial in a manner dependent on CNT length, as described above.Infused CNTs can vary in length ranging from between about 1 micron toabout 500 microns, including 1 micron, 2 microns, 3 microns, 4 micron,5, microns, 6, microns, 7 microns, 8 microns, 9 microns, 10 microns, 15microns, 20 microns, 25 microns, 30 microns, 35 microns, 40 microns, 45microns, 50 microns, 60 microns, 70 microns, 80 microns, 90 microns, 100microns, 150 microns, 200 microns, 250 microns, 300 microns, 350microns, 400 microns, 450 microns, 500 microns, and all values inbetween. CNTs can also be less than about 1 micron in length, includingabout 0.5 microns, for example. CNTs can also be greater than 500microns, including for example, 510 microns, 520 microns, 550 microns,600 microns, 700 microns and all values in between.

Compositions of the invention can incorporate CNTs have a length fromabout 1 micron to about 10 microns. Such CNT lengths can be useful inapplication to increase shear strength. CNTs can also have a length fromabout 5 to about 70 microns. Such CNT lengths can be useful inapplications for increased tensile strength if the CNTs are aligned inthe fiber direction. CNTs can also have a length from about 10 micronsto about 100 microns. Such CNT lengths can be useful to increaseelectrical/thermal properties as well as mechanical properties. Theprocess used in the invention can also provide CNTs having a length fromabout 100 microns to about 500 microns, which can also be beneficial toincrease electrical and thermal properties. Such control of CNT lengthis readily achieved through modulation of carbon feedstock and inert gasflow rates coupled with varying linespeeds and growth temperature.

In some embodiments, compositions that include spoolable lengths ofCNT-infused carbon fiber materials can have various uniform regions withdifferent lengths of CNTs. For example, it can be desirable to have afirst portion of CNT-infused carbon fiber material with uniformlyshorter CNT lengths to enhance shear strength properties, and a secondportion of the same spoolable material with a uniform longer CNT lengthto enhance electrical or thermal properties.

Processes of the invention for CNT infusion to carbon fiber materialsallow control of the CNT lengths with uniformity and in a continuousprocess allowing spoolable carbon fiber materials to be functionalizedwith CNTs at high rates. With material residence times between 5 to 300seconds, linespeeds in a continuous process for a system that is 3 feetlong can be in a range anywhere from about 0.5 ft/min to about 36 ft/minand greater. The speed selected depends on various parameters asexplained further below.

In some embodiments, a material residence time of about 5 to about 30seconds can produce CNTs having a length between about 1 micron to about10 microns. In some embodiments, a material residence time of about 30to about 180 seconds can produce CNTs having a length between about 10microns to about 100 microns. In still further embodiments, a materialresidence time of about 180 to about 300 seconds can produce CNTs havinga length between about 100 microns to about 500 microns. One skilled inthe art will recognize that these ranges are approximate and that CNTlength can also be modulated by reaction temperatures, and carrier andcarbon feedstock concentrations and flow rates.

CNT-infused carbon fiber materials of the invention include a barriercoating. Barrier coatings can include for example an alkoxysilane,methylsiloxane, an alumoxane, alumina nanoparticles, spin on glass andglass nanoparticles. As described below, the CNT-forming catalyst can beadded to the uncured barrier coating material and then applied to thecarbon fiber material together. In other embodiments the barrier coatingmaterial can be added to the carbon fiber material prior to depositionof the CNT-forming catalyst. The barrier coating material can be of athickness sufficiently thin to allow exposure of the CNT-formingcatalyst to the carbon feedstock for subsequent CVD growth. In someembodiments, the thickness is less than or about equal to the effectivediameter of the CNT-forming catalyst. In some embodiments, the thicknessof the barrier coating is in a range from between about 10 nm to about100 nm. The barrier coating can also be less than 10 nm, including 1 nm,2 nm, 3 nm, 4 nm, 5 nm, 6 nm, 7 nm, 8 nm, 9 nm, 10 nm, and any value inbetween.

Without being bound by theory, the barrier coating can serve as anintermediate layer between the carbon fiber material and the CNTs andserves to mechanically infuse the CNTs to the carbon fiber material.Such mechanical infusion still provides a robust system in which thecarbon fiber material serves as a platform for organizing the CNTs whilestill imparting properties of the CNTs to the carbon fiber material.Moreover, the benefit of including a barrier coating is the immediateprotection it provides the carbon fiber material from chemical damagedue to exposure to moisture and/or any thermal damage due to heating ofthe carbon fiber material at the temperatures used to promote CNTgrowth.

The infused CNTs disclosed herein can effectively function as areplacement for conventional carbon fiber “sizing.” The infused CNTs aremore robust than conventional sizing materials and can improve thefiber-to-matrix interface in composite materials and, more generally,improve fiber-to-fiber interfaces. Indeed, the CNT-infused carbon fibermaterials disclosed herein are themselves composite materials in thesense the CNT-infused carbon fiber material properties will be acombination of those of the carbon fiber material as well as those ofthe infused CNTs. Consequently, embodiments of the present inventionprovide a means to impart desired properties to a carbon fiber materialthat otherwise lack such properties or possesses them in insufficientmeasure. Carbon fiber materials can be tailored or engineered to meetthe requirements of specific applications. The CNTs acting as sizing canprotect carbon fiber materials from absorbing moisture due to thehydrophobic CNT structure. Moreover, hydrophobic matrix materials, asfurther exemplified below, interact well with hydrophobic CNTs toprovide improved fiber to matrix interactions.

Despite the beneficial properties imparted to a carbon fiber materialhaving infused CNTs described above, the compositions of the presentinvention can include further “conventional” sizing agents. Such sizingagents vary widely in type and function and include, for example,surfactants, anti-static agents, lubricants, siloxanes, alkoxysilanes,aminosilanes, silanes, silanols, polyvinyl alcohol, starch, and mixturesthereof. Such secondary sizing agents can be used to protect the CNTsthemselves or provide further properties to the fiber not imparted bythe presence of the infused CNTs.

Compositions of the present invention can further include a matrixmaterial to form a composite with the CNT-infused carbon fiber material.Such matrix materials can include, for example, an epoxy, a polyester, avinylester, a polyetherimide, a polyetherketoneketone, apolyphthalamide, a polyetherketone, a polytheretherketone, a polyimide,a phenol-formaldehyde, and a bismaleimide. Matrix materials useful inthe present invention can include any of the known matrix materials (seeMel M. Schwartz, Composite Materials Handbook (2d ed. 1992)). Matrixmaterials more generally can include resins (polymers), boththermosetting and thermoplastic, metals, ceramics, and cements.

Thermosetting resins useful as matrix materials include phthalic/maleictype polyesters, vinyl esters, epoxies, phenolics, cyanates,bismaleimides, and nadic end-capped polyimides (e.g., PMR-15).Thermoplastic resins include polysulfones, polyamides, polycarbonates,polyphenylene oxides, polysulfides, polyether ether ketones, polyethersulfones, polyamide-imides, polyetherimides, polyimides, polyarylates,and liquid crystalline polyester.

Metals useful as matrix materials include alloys of aluminum such asaluminum 6061, 2024, and 713 aluminum braze. Ceramics useful as matrixmaterials include carbon ceramics, such as lithium aluminosilicate,oxides such as alumina and mullite, nitrides such as silicon nitride,and carbides such as silicon carbide. Cements useful as matrix materialsinclude carbide-base cermets (tungsten carbide, chromium carbide, andtitanium carbide), refractory cements (tungsten-thoria andbarium-carbonate-nickel), chromium-alumina, nickel-magnesiairon-zirconium carbide. Any of the above-described matrix materials canbe used alone or in combination.

FIG. 1-6 shows TEM and SEM images of carbon fiber materials prepared bythe processes described herein. The procedures for preparing thesematerials are further detailed below and in Examples I-III. FIGS. 1 and2 show TEM images of multi-walled and double-walled carbon nanotubes,respectively, that were prepared on an AS4 carbon fiber in a continuousprocess. FIG. 3 shows a scanning electron microscope (SEM) image of CNTsgrowing from within the barrier coating after the CNT-formingnanoparticle catalyst was mechanically infused to a carbon fibermaterial surface. FIG. 4 shows a SEM image demonstrating the consistencyin length distribution of CNTs grown on a carbon fiber material towithin 20% of a targeted length of about 40 microns. FIG. 5 shows an SEMimage demonstrating the effect of a barrier coating on CNT growth.Dense, well aligned CNTs grew where barrier coating was applied and noCNTs grew where barrier coating was absent. FIG. 6 shows a lowmagnification SEM of CNTs on carbon fiber demonstrating the uniformityof CNT density across the fibers within about 10%.

CNT-infused carbon fiber materials can be used in a myriad ofapplications. For example, chopped CNT-infused carbon fiber can be usedin propellant applications. U.S. Pat. No. 4,072,546 describes the use ofgraphite fibers to augment propellant burning rate. The presence of CNTsinfused on chopped carbon fiber can further enhance such burn rates.CNT-infused carbon fiber materials can also be used in flame retardantapplications as well. For example, the CNTs can form a protective charlayer that retards burning of a material coated with a layer of CNTinfused carbon fiber material.

CNT-infused conductive carbon fibers can be used in the manufacture ofelectrodes for superconductors. In the production of superconductingfibers, it can be challenging to achieve adequate adhesion of thesuperconducting layer to a carrier fiber due, in part, to the differentcoefficients of thermal expansion of the fiber material and of thesuperconducting layer. Another difficulty in the art arises during thecoating of the fibers by the CVD process. For example, reactive gases,such as hydrogen gas or ammonia, can attack the fiber surface and/orform undesired hydrocarbon compounds on the fiber surface and make goodadhesion of the superconducting layer more difficult. CNT-infused carbonfiber materials with barrier coating can overcome these aforementionedchallenges in the art.

CNT infused carbon fiber materials can be used in applications requiringwear-resistance. U.S. Pat. No. 6,691,393 describes wear resistance incarbon fiber friction materials. Such carbon fiber friction materialsare used in, for example, automotive brake discs. Other wear resistanceapplications can include, for example, rubber o-rings and gasket seals.

The large effective surface area of CNTs makes the CNT-infused carbonfiber materials effective for water filtration applications and otherextractive processes, such as separation of organic oils from water.CNT-infused carbon fiber materials can be used to remove organic toxinsfrom water tables, water storage facilities, or in-line filters for homeand office use.

In oilfield technologies, the CNT-infused carbon fibers are useful inthe manufacture of drilling equipment, such as pipe bearings, pipingreinforcement, and rubber o-rings. Furthermore, as described above,CNT-infused carbon fibers can be used in extractive processes. Applyingsuch extraction properties in a formation containing valuable petroleumdeposits, the CNT-infused carbon fiber materials can be used to extractoil from otherwise intractable formations. For example, the CNT-infusecarbon fiber materials can be used to extract oil from formations wheresubstantial water and/or sand is present. The CNT-infused carbon fibermaterial can also be useful to extract heavier oils that would otherwisebe difficult to extract due to their high boiling points. In conjunctionwith a perforated piping system, for example, the wicking of such heavyoils by CNT-infused carbon materials overcoated on the perforated pipingcan be operatively coupled to a vacuum system, or the like, tocontinuously remove high boiling fractions from heavy oil and oil shaleformations. Moreover, such processes can be used in conjunction with, orin lieu, of conventional thermal or catalyzed cracking methods, known inthe art.

CNT-infused carbon fiber materials can enhance structural elements inaerospace and ballistics applications. For example, the structures suchas nose cones in missiles, leading edge of wings, primary structuralparts, such as flaps and aerofoils, propellers and air brakes, smallplane fuselages, helicopter shells and rotor blades, aircraft secondarystructural parts, such as floors, doors, seats, air conditioners, andsecondary tanks and airplane motor parts can benefit from the structuralenhancement provided by CNT-infused carbon fibers. Structuralenhancement in many other applications can include, for example, minesweeper hulls, helmets, radomes, rocket nozzles, rescue stretchers, andengine components. In building and construction, structural enhancementof exterior features include columns, pediments, domes, cornices, andformwork. Likewise, in interior building structures such as blinds,sanitary-ware, window profiles, and the like can all benefit from theuse of CNT-infused carbon fiber materials.

In maritime industry, structural enhancement can include boat hulls,stringers, and decks. CNT-infused carbon fiber materials can also beused in the heavy transportation industry in large panels for trailerwalls, floor panels for railcars, truck cabs, exterior body molding, busbody shells, and cargo containers, for example. In automotiveapplications, CNT-infused carbon fiber materials can be used in interiorparts, such as trimming, seats, and instrument panels. Exteriorstructures such as body panels, openings, underbody, and front and rearmodules can all benefit from the use of CNT-infused carbon fibermaterials. Even automotive engine compartment and fuel mechanical areaparts, such as axles and suspensions, fuel and exhaust systems, andelectrical and electronic components can all utilize CNT-infused carbonfiber materials.

Other applications of CNT-infused carbon fiber materials include, bridgeconstruction, reinforced concrete products, such as dowel bars,reinforcing bars, post-tensioning and pre-stressing tendons,stay-in-place framework, electric power transmission and distributionstructures such as utility poles, transmission poles, and cross-arms,highway safety and roadside features such as sign supports, guardrails,posts and supports, noise barriers, and in municipal pipes and storagetanks

CNT-infused carbon fiber materials can also be used in a variety ofleisure equipment such as water and snow skis, kayaks, canoes andpaddles, snowboards, golf club shafts, golf trolleys, fishing rods, andswimming pools. Other consumer goods and business equipment includegears, pans, housings, gas pressure bottles, components for householdappliances, such as washers, washing machine drums, dryers, wastedisposal units, air conditioners and humidifiers.

The electrical properties of CNT-infused carbon fibers also can impactvarious energy and electrical applications. For example, CNT-infusedcarbon fiber materials can be used in wind turbine blades, solarstructures, electronic enclosures, such as laptops, cell phones,computer cabinets, where such CNT-infused materials can be used in EMIshielding, for example. Other applications include powerlines, coolingdevices, light poles, circuit boards, electrical junction boxes, ladderrails, optical fiber, power built into structures such as data lines,computer terminal housings, and business equipment, such as copiers,cash registers and mailing equipment.

In some embodiments the present invention provides a continuous processfor CNT infusion that includes (a) disposing a carbon nanotube-formingcatalyst on a surface of a carbon fiber material of spoolabledimensions; and (b) synthesizing carbon nanotubes directly on the carbonfiber material, thereby forming a carbon nanotube-infused carbon fibermaterial. For a 9 foot long system, the linespeed of the process canrange from between about 1.5 ft/min to about 108 ft/min. The linespeedsachieved by the process described herein allow the formation ofcommercially relevant quantities of CNT-infused carbon fiber materialswith short production times. For example, at 36 ft/min linespeed, thequantities of CNT-infused carbon fibers (over 5% infused CNTs on fiberby weight) can exceed over 100 pound or more of material produced perday in a system that is designed to simultaneously process 5 separatetows (20 lb/tow). Systems can be made to produce more tows at once or atfaster speeds by repeating growth zones. Moreover, some steps in thefabrication of CNTs, as known in the art, have prohibitively slow ratespreventing a continuous mode of operation. For example, in a typicalprocess known in the art, a CNT-forming catalyst reduction step can take1-12 hours to perform. CNT growth itself can also be time consuming, forexample requiring tens of minutes for CNT growth, precluding the rapidlinespeeds realized in the present invention. The process describedherein overcomes such rate limiting steps.

The CNT-infused carbon fiber material-forming processes of the inventioncan avoid CNT entanglement that occurs when trying to apply suspensionsof pre-formed carbon nanotubes to fiber materials. That is, becausepre-formed CNTs are not fused to the carbon fiber material, the CNTstend to bundle and entangle. The result is a poorly uniform distributionof CNTs that weakly adhere to the carbon fiber material. However,processes of the present invention can provide, if desired, a highlyuniform entangled CNT mat on the surface of the carbon fiber material byreducing the growth density. The CNTs grown at low density are infusedin the carbon fiber material first. In such embodiments, the fibers donot grow dense enough to induce vertical alignment, the result isentangled mats on the carbon fiber material surfaces. By contrast,manual application of pre-formed CNTs does not insure uniformdistribution and density of a CNT mat on the carbon fiber material.

FIG. 7 depicts a flow diagram of process 700 for producing CNT-infusedcarbon fiber material in accordance with an illustrative embodiment ofthe present invention.

Process 700 includes at least the operations of:

701: Functionalizing the carbon fiber material.

702: Applying a barrier coating and a CNT-forming catalyst to thefunctionalized carbon fiber material.

704: Heating the carbon fiber material to a temperature that issufficient for carbon nanotube synthesis.

706: Promoting CVD-mediated CNT growth on the catalyst-laden carbonfiber.

In step 701, the carbon fiber material is functionalized to promotesurface wetting of the fibers and to improve adhesion of the barriercoating.

To infuse carbon nanotubes into a carbon fiber material, the carbonnanotubes are synthesized on the carbon fiber material which isconformally coated with a barrier coating. In one embodiment, this isaccomplished by first conformally coating the carbon fiber material witha barrier coating and then disposing nanotube-forming catalyst on thebarrier coating, as per operation 702. In some embodiments, the barriercoating can be partially cured prior to catalyst deposition. This canprovide a surface that is receptive to receiving the catalyst andallowing it to embed in the barrier coating, including allowing surfacecontact between the CNT forming catalyst and the carbon fiber material.In such embodiments, the barrier coating can be fully cured afterembedding the catalyst. In some embodiments, the barrier coating isconformally coated over the carbon fiber material simultaneously withdeposition of the CNT-form catalyst. Once the CNT-forming catalyst andbarrier coating are in place, the barrier coating can be fully cured.

In some embodiments, the barrier coating can be fully cured prior tocatalyst deposition. In such embodiments, a fully cured barrier-coatedcarbon fiber material can be treated with a plasma to prepare thesurface to accept the catalyst. For example, a plasma treated carbonfiber material having a cured barrier coating can provide a roughenedsurface in which the CNT-forming catalyst can be deposited. The plasmaprocess for “roughing” the surface of the barrier thus facilitatescatalyst deposition. The roughness is typically on the scale ofnanometers. In the plasma treatment process craters or depressions areformed that are nanometers deep and nanometers in diameter. Such surfacemodification can be achieved using a plasma of any one or more of avariety of different gases, including, without limitation, argon,helium, oxygen, nitrogen, and hydrogen. In some embodiments, plasmaroughing can also be performed directly in the carbon fiber materialitself. This can facilitate adhesion of the barrier coating to thecarbon fiber material.

As described further below and in conjunction with FIG. 7, the catalystis prepared as a liquid solution that contains CNT-forming catalyst thatcomprise transition metal nanoparticles. The diameters of thesynthesized nanotubes are related to the size of the metal particles asdescribed above. In some embodiments, commercial dispersions ofCNT-forming transition metal nanoparticle catalyst are available and areused without dilution, in other embodiments commercial dispersions ofcatalyst can be diluted. Whether to dilute such solutions can depend onthe desired density and length of CNT to be grown as described above.

With reference to the illustrative embodiment of FIG. 7, carbon nanotubesynthesis is shown based on a chemical vapor deposition (CVD) processand occurs at elevated temperatures. The specific temperature is afunction of catalyst choice, but will typically be in a range of about500 to 1000° C. Accordingly, operation 704 involves heating thebarrier-coated carbon fiber material to a temperature in theaforementioned range to support carbon nanotube synthesis.

In operation 706, CVD-promoted nanotube growth on the catalyst-ladencarbon fiber material is then performed. The CVD process can be promotedby, for example, a carbon-containing feedstock gas such as acetylene,ethylene, and/or ethanol. The CNT synthesis processes generally use aninert gas (nitrogen, argon, helium) as a primary carrier gas. The carbonfeedstock is provided in a range from between about 0% to about 15% ofthe total mixture. A substantially inert environment for CVD growth isprepared by removal of moisture and oxygen from the growth chamber.

In the CNT synthesis process, CNTs grow at the sites of a CNT-formingtransition metal nanoparticle catalyst. The presence of the strongplasma-creating electric field can be optionally employed to affectnanotube growth. That is, the growth tends to follow the direction ofthe electric field. By properly adjusting the geometry of the plasmaspray and electric field, vertically-aligned CNTs (i.e., perpendicularto the carbon fiber material) can be synthesized. Under certainconditions, even in the absence of a plasma, closely-spaced nanotubeswill maintain a vertical growth direction resulting in a dense array ofCNTs resembling a carpet or forest. The presence of the barrier coatingcan also influence the directionality of CNT growth.

The operation of disposing a catalyst on the carbon fiber material canbe accomplished by spraying or dip coating a solution or by gas phasedeposition via, for example, a plasma process. The choice of techniquescan be coordinated with the mode with which the barrier coating isapplied. Thus, in some embodiments, after forming a solution of acatalyst in a solvent, catalyst can be applied by spraying or dipcoating the barrier coated carbon fiber material with the solution, orcombinations of spraying and dip coating. Either technique, used aloneor in combination, can be employed once, twice, thrice, four times, upto any number of times to provide a carbon fiber material that issufficiently uniformly coated with CNT-forming catalyst. When dipcoating is employed, for example, a carbon fiber material can be placedin a first dip bath for a first residence time in the first dip bath.When employing a second dip bath, the carbon fiber material can beplaced in the second dip bath for a second residence time. For example,carbon fiber materials can be subjected to a solution of CNT-formingcatalyst for between about 3 seconds to about 90 seconds depending onthe dip configuration and linespeed. Employing spraying or dip coatingprocesses, a carbon fiber material with a surface density of catalyst ofless than about 5% surface coverage to as high as about 80% coverage, inwhich the CNT-forming catalyst nanoparticles are nearly monolayer. Insome embodiments, the process of coating the CNT-forming catalyst on thecarbon fiber material should produce no more than a monolayer. Forexample, CNT growth on a stack of CNT-forming catalyst can erode thedegree of infusion of the CNT to the carbon fiber material. In otherembodiments, the transition metal catalyst can be deposited on thecarbon fiber material using evaporation techniques, electrolyticdeposition techniques, and other processes known to those skilled in theart, such as addition of the transition metal catalyst to a plasmafeedstock gas as a metal organic, metal salt or other compositionpromoting gas phase transport.

Because processes of the invention are designed to be continuous, aspoolable carbon fiber material can be dip-coated in a series of bathswhere dip coating baths are spatially separated. In a continuous processin which nascent carbon fibers are being generated de novo, dip bath orspraying of CNT-forming catalyst can be the first step after applyingand curing or partially curing a barrier coating to the carbon fibermaterial. Application of the barrier coating and a CNT-forming catalystcan be performed in lieu of application of a sizing, for newly formedcarbon fiber materials. In other embodiments, the CNT-forming catalystcan be applied to newly formed carbon fibers in the presence of othersizing agents after barrier coating. Such simultaneous application ofCNT-forming catalyst and other sizing agents can still provide theCNT-forming catalyst in surface contact with the barrier coating of thecarbon fiber material to insure CNT infusion.

The catalyst solution employed can be a transition metal nanoparticlewhich can be any d-block transition metal as described above. Inaddition, the nanoparticles can include alloys and non-alloy mixtures ofd-block metals in elemental form or in salt form, and mixtures thereof.Such salt forms include, without limitation, oxides, carbides, andnitrides. Non-limiting exemplary transition metal NPs include Ni, Fe,Co, Mo, Cu, Pt, Au, and Ag and salts thereof and mixtures thereof. Insome embodiments, such CNT-forming catalysts are disposed on the carbonfiber by applying or infusing a CNT-forming catalyst directly to thecarbon fiber material simultaneously with barrier coating deposition.Many of these transition metal catalysts are readily commerciallyavailable from a variety of suppliers, including, for example, FerrotecCorporation (Bedford, N.H.).

Catalyst solutions used for applying the CNT-forming catalyst to thecarbon fiber material can be in any common solvent that allows theCNT-forming catalyst to be uniformly dispersed throughout. Such solventscan include, without limitation, water, acetone, hexane, isopropylalcohol, toluene, ethanol, methanol, tetrahydrofuran (THF), cyclohexaneor any other solvent with controlled polarity to create an appropriatedispersion of the CNT-forming catalyst nanoparticles. Concentrations ofCNT-forming catalyst can be in a range from about 1:1 to 1:10000catalyst to solvent. Such concentrations can be used when the barriercoating and CNT-forming catalyst is applied simultaneously as well.

In some embodiments heating of the carbon fiber material can be at atemperature that is between about 500° C. and 1000° C. to synthesizecarbon nanotubes after deposition of the CNT-forming catalyst. Heatingat these temperatures can be performed prior to or substantiallysimultaneously with introduction of a carbon feedstock for CNT growth.

In some embodiments, the present invention provides a process thatincludes removing sizing agents from a carbon fiber material, applying abarrier coating conformally over the carbon fiber material, applying aCNT-forming catalyst to the carbon fiber material, heating the carbonfiber material to at least 500° C., and synthesizing carbon nanotubes onthe carbon fiber material. In some embodiments, operations of theCNT-infusion process include removing sizing from a carbon fibermaterial, applying a barrier coating to the carbon fiber material,applying a CNT-forming catalyst to the carbon fiber, heating the fiberto CNT-synthesis temperature and CVD-promoted CNT growth thecatalyst-laden carbon fiber material. Thus, where commercial carbonfiber materials are employed, processes for constructing CNT-infusedcarbon fibers can include a discrete step of removing sizing from thecarbon fiber material before disposing barrier coating and the catalyston the carbon fiber material.

The step of synthesizing carbon nanotubes can include numeroustechniques for forming carbon nanotubes, including those disclosed inco-pending U.S. Patent Application No. US 2004/0245088 which isincorporated herein by reference. The CNTs grown on fibers of thepresent invention can be accomplished by techniques known in the artincluding, without limitation, micro-cavity, thermal or plasma-enhancedCVD techniques, laser ablation, arc discharge, and high pressure carbonmonoxide (HiPCO). During CVD, in particular, a barrier coated carbonfiber material with CNT-forming catalyst disposed thereon, can be useddirectly. In some embodiments, any conventional sizing agents can beremoved prior CNT synthesis. In some embodiments, acetylene gas isionized to create a jet of cold carbon plasma for CNT synthesis. Theplasma is directed toward the catalyst-bearing carbon fiber material.Thus, in some embodiments synthesizing CNTs on a carbon fiber materialincludes (a) forming a carbon plasma; and (b) directing the carbonplasma onto the catalyst disposed on the carbon fiber material. Thediameters of the CNTs that are grown are dictated by the size of theCNT-forming catalyst as described above. In some embodiments, the sizedfiber substrate is heated to between about 550 to about 800° C. tofacilitate CNT synthesis. To initiate the growth of CNTs, two gases arebled into the reactor: a process gas such as argon, helium, or nitrogen,and a carbon-containing gas, such as acetylene, ethylene, ethanol ormethane. CNTs grow at the sites of the CNT-forming catalyst.

In some embodiments, the CVD growth is plasma-enhanced. A plasma can begenerated by providing an electric field during the growth process. CNTsgrown under these conditions can follow the direction of the electricfield. Thus, by adjusting the geometry of the reactor vertically alignedcarbon nanotubes can be grown radially about a cylindrical fiber. Insome embodiments, a plasma is not required for radial growth about thefiber. For carbon fiber materials that have distinct sides such astapes, mats, fabrics, plies, and the like, catalyst can be disposed onone or both sides and correspondingly, CNTs can be grown on one or bothsides as well.

As described above, CNT-synthesis is performed at a rate sufficient toprovide a continuous process for functionalizing spoolable carbon fibermaterials. Numerous apparatus configurations facilitate such continuoussynthesis as exemplified below.

In some embodiments, CNT-infused carbon fiber materials can beconstructed in an “all plasma” process. An all plasma process can beingwith roughing the carbon fiber material with a plasma as described aboveto improve fiber surface wetting characteristics and provide a moreconformal barrier coating, as well as improve coating adhesion viamechanical interlocking and chemical adhesion through the use offunctionalization of the carbon fiber material by using specificreactive gas species, such as oxygen, nitrogen, hydrogen in argon orhelium based plasmas.

Barrier coated carbon fiber materials pass through numerous furtherplasma-mediated steps to form the final CNT-infused product. In someembodiments, the all plasma process can include a second surfacemodification after the barrier coating is cured. This is a plasmaprocess for “roughing” the surface of the barrier coating on the carbonfiber material to facilitate catalyst deposition. As described above,surface modification can be achieved using a plasma of any one or moreof a variety of different gases, including, without limitation, argon,helium, oxygen, ammonia, hydrogen, and nitrogen.

After surface modification, the barrier coated carbon fiber materialproceeds to catalyst application. This is a plasma process fordepositing the CNT-forming catalyst on the fibers. The CNT-formingcatalyst is typically a transition metal as described above. Thetransition metal catalyst can be added to a plasma feedstock gas as aprecursor in the form of a ferrofluid, a metal organic, metal salt orother composition for promoting gas phase transport. The catalyst can beapplied at room temperature in the ambient environment with neithervacuum nor an inert atmosphere being required. In some embodiments, thecarbon fiber material is cooled prior to catalyst application.

Continuing the all-plasma process, carbon nanotube synthesis occurs in aCNT-growth reactor. This can be achieved through the use ofplasma-enhanced chemical vapor deposition, wherein carbon plasma issprayed onto the catalyst-laden fibers. Since carbon nanotube growthoccurs at elevated temperatures (typically in a range of about 500 to1000° C. depending on the catalyst), the catalyst-laden fibers can beheated prior to exposing to the carbon plasma. For the infusion process,the carbon fiber material can be optionally heated until it softens.After heating, the carbon fiber material is ready to receive the carbonplasma. The carbon plasma is generated, for example, by passing a carboncontaining gas such as acetylene, ethylene, ethanol, and the like,through an electric field that is capable of ionizing the gas. This coldcarbon plasma is directed, via spray nozzles, to the carbon fibermaterial. The carbon fiber material can be in close proximity to thespray nozzles, such as within about 1 centimeter of the spray nozzles,to receive the plasma. In some embodiments, heaters are disposed abovethe carbon fiber material at the plasma sprayers to maintain theelevated temperature of the carbon fiber material.

Another configuration for continuous carbon nanotube synthesis involvesa special rectangular reactor for the synthesis and growth of carbonnanotubes directly on carbon fiber materials. The reactor can bedesigned for use in a continuous in-line process for producingcarbon-nanotube bearing fibers. In some embodiments, CNTs are grown viaa chemical vapor deposition (“CVD”) process at atmospheric pressure andat elevated temperature in the range of about 550° C. to about 800° C.in a multi-zone reactor. The fact that the synthesis occurs atatmospheric pressure is one factor that facilitates the incorporation ofthe reactor into a continuous processing line for CNT-on-fibersynthesis. Another advantage consistent with in-line continuousprocessing using such a zone reactor is that CNT growth occurs in aseconds, as opposed to minutes (or longer) as in other procedures andapparatus configurations typical in the art.

CNT synthesis reactors in accordance with the various embodimentsinclude the following features:

Rectangular Configured Synthesis Reactors:

The cross section of a typical CNT synthesis reactor known in the art iscircular. There are a number of reasons for this including, for example,historical reasons (cylindrical reactors are often used in laboratories)and convenience (flow dynamics are easy to model in cylindricalreactors, heater systems readily accept circular tubes (quartz, etc.),and ease of manufacturing. Departing from the cylindrical convention,the present invention provides a CNT synthesis reactor having arectangular cross section. The reasons for the departure are asfollows: 1. Since many carbon fiber materials that can be processed bythe reactor are relatively planar such as flat tape or sheet-like inform, a circular cross section is an inefficient use of the reactorvolume. This inefficiency results in several drawbacks for cylindricalCNT synthesis reactors including, for example, a) maintaining asufficient system purge; increased reactor volume requires increased gasflow rates to maintain the same level of gas purge. This results in asystem that is inefficient for high volume production of CNTs in an openenvironment; b) increased carbon feedstock gas flow; the relativeincrease in inert gas flow, as per a) above, requires increased carbonfeedstock gas flows. Consider that the volume of a 12K carbon fiber towis 2000 times less than the total volume of a synthesis reactor having arectangular cross section. In an equivalent growth cylindrical reactor(i.e., a cylindrical reactor that has a width that accommodates the sameplanarized carbon fiber material as the rectangular cross-sectionreactor), the volume of the carbon fiber material is 17,500 times lessthan the volume of the chamber. Although gas deposition processes, suchas CVD, are typically governed by pressure and temperature alone, volumehas a significant impact on the efficiency of deposition. With arectangular reactor there is a still excess volume. This excess volumefacilitates unwanted reactions; yet a cylindrical reactor has abouteight times that volume. Due to this greater opportunity for competingreactions to occur, the desired reactions effectively occur more slowlyin a cylindrical reactor chamber. Such a slow down in CNT growth, isproblematic for the development of a continuous process. One benefit ofa rectangular reactor configuration is that the reactor volume can bedecreased by using a small height for the rectangular chamber to makethis volume ratio better and reactions more efficient. In someembodiments of the present invention, the total volume of a rectangularsynthesis reactor is no more than about 3000 times greater than thetotal volume of a carbon fiber material being passed through thesynthesis reactor. In some further embodiments, the total volume of therectangular synthesis reactor is no more than about 4000 times greaterthan the total volume of the carbon fiber material being passed throughthe synthesis reactor. In some still further embodiments, the totalvolume of the rectangular synthesis reactor is less than about 10,000times greater than the total volume of the carbon fiber material beingpassed through the synthesis reactor. Additionally, it is notable thatwhen using a cylindrical reactor, more carbon feedstock gas is requiredto provide the same flow percent as compared to reactors having arectangular cross section. It should be appreciated that in some otherembodiments, the synthesis reactor has a cross section that is describedby polygonal forms that are not rectangular, but are relatively similarthereto and provide a similar reduction in reactor volume relative to areactor having a circular cross section; c) problematic temperaturedistribution; when a relatively small-diameter reactor is used, thetemperature gradient from the center of the chamber to the walls thereofis minimal. But with increased size, such as would be used forcommercial-scale production, the temperature gradient increases. Suchtemperature gradients result in product quality variations across acarbon fiber material substrate (i.e., product quality varies as afunction of radial position). This problem is substantially avoided whenusing a reactor having a rectangular cross section. In particular, whena planar substrate is used, reactor height can be maintained constant asthe size of the substrate scales upward. Temperature gradients betweenthe top and bottom of the reactor are essentially negligible and, as aconsequence, thermal issues and the product-quality variations thatresult are avoided. 2. Gas introduction: Because tubular furnaces arenormally employed in the art, typical CNT synthesis reactors introducegas at one end and draw it through the reactor to the other end. In someembodiments disclosed herein, gas can be introduced at the center of thereactor or within a target growth zone, symmetrically, either throughthe sides or through the top and bottom plates of the reactor. Thisimproves the overall CNT growth rate because the incoming feedstock gasis continuously replenishing at the hottest portion of the system, whichis where CNT growth is most active. This constant gas replenishment isan important aspect to the increased growth rate exhibited by therectangular CNT reactors.

Zoning.

Chambers that provide a relatively cool purge zone depend from both endsof the rectangular synthesis reactor. Applicants have determined that ifhot gas were to mix with the external environment (i.e., outside of thereactor), there would be an increase in degradation of the carbon fibermaterial. The cool purge zones provide a buffer between the internalsystem and external environments. Typical CNT synthesis reactorconfigurations known in the art typically require that the substrate iscarefully (and slowly) cooled. The cool purge zone at the exit of thepresent rectangular CNT growth reactor achieves the cooling in a shortperiod of time, as required for the continuous in-line processing.

Non-Contact, Hot-Walled, Metallic Reactor.

In some embodiments, a hot-walled reactor is made of metal is employed,in particular stainless steel. This may appear counterintuitive becausemetal, and stainless steel in particular, is more susceptible to carbondeposition (i.e., soot and by-product formation). Thus, most CNT reactorconfigurations use quartz reactors because there is less carbondeposited, quartz is easier to clean, and quartz facilitates sampleobservation. However, Applicants have observed that the increased sootand carbon deposition on stainless steel results in more consistent,faster, more efficient, and more stable CNT growth. Without being boundby theory it has been indicated that, in conjunction with atmosphericoperation, the CVD process occurring in the reactor is diffusionlimited. That is, the catalyst is “overfed;” too much carbon isavailable in the reactor system due to its relatively higher partialpressure (than if the reactor was operating under partial vacuum). As aconsequence, in an open system—especially a clean one—too much carboncan adhere to catalyst particles, compromising their ability tosynthesize CNTs. In some embodiments, the rectangular reactor isintentionally run when the reactor is “dirty,” that is with sootdeposited on the metallic reactor walls. Once carbon deposits to amonolayer on the walls of the reactor, carbon will readily deposit overitself. Since some of the available carbon is “withdrawn” due to thismechanism, the remaining carbon feedstock, in the form of radicals,react with the catalyst at a rate that does not poison the catalyst.Existing systems run “cleanly” which, if they were open for continuousprocessing, would produced a much lower yield of CNTs at reduced growthrates.

Although it is generally beneficial to perform CNT synthesis “dirty” asdescribed above, certain portions of the apparatus, such as gasmanifolds and inlets, can nonetheless negatively impact the CNT growthprocess when soot created blockages. In order to combat this problem,such areas of the CNT growth reaction chamber can be protected with sootinhibiting coatings such as silica, alumina, or MgO. In practice, theseportions of the apparatus can be dip-coated in these soot inhibitingcoatings. Metals such as INVAR® can be used with these coatings as INVARhas a similar CTE (coefficient of thermal expansion) ensuring properadhesion of the coating at higher temperatures, preventing the soot fromsignificantly building up in critical zones.

Combined Catalyst Reduction and CNT Synthesis.

In the CNT synthesis reactor disclosed herein, both catalyst reductionand CNT growth occur within the reactor. This is significant because thereduction step cannot be accomplished timely enough for use in acontinuous process if performed as a discrete operation. In a typicalprocess known in the art, a reduction step typically takes 1-12 hours toperform. Both operations occur in a reactor in accordance with thepresent invention due, at least in part, to the fact that carbonfeedstock gas is introduced at the center of the reactor, not the end aswould be typical in the art using cylindrical reactors. The reductionprocess occurs as the fibers enter the heated zone; by this point, thegas has had time to react with the walls and cool off prior to reactingwith the catalyst and causing the oxidation reduction (via hydrogenradical interactions). It is this transition region where the reductionoccurs. At the hottest isothermal zone in the system, the CNT growthoccurs, with the greatest growth rate occurring proximal to the gasinlets near the center of the reactor.

In some embodiments, when loosely affiliated carbon fiber materials,such as carbon tow are employed, the continuous process can includesteps that spreads out the strands and/or filaments of the tow. Thus, asa tow is unspooled it can be spread using a vacuum-based fiber spreadingsystem, for example. When employing sized carbon fibers, which can berelatively stiff, additional heating can be employed in order to“soften” the tow to facilitate fiber spreading. The spread fibers whichcomprise individual filaments can be spread apart sufficiently to exposean entire surface area of the filaments, thus allowing the tow to moreefficiently react in subsequent process steps. Such spreading canapproach between about 4 inches to about 6 inches across for a 3 k tow.The spread carbon tow can pass through a surface treatment step that iscomposed of a plasma system as described above. After a barrier coatingis applied and roughened, spread fibers then can pass through aCNT-forming catalyst dip bath. The result is fibers of the carbon towthat have catalyst particles distributed radially on their surface. Thecatalyzed-laden fibers of the tow then enter an appropriate CNT growthchamber, such as the rectangular chamber described above, where a flowthrough atmospheric pressure CVD or PE-CVD process is used to synthesizethe CNTs at rates as high as several microns per second. The fibers ofthe tow, now with radially aligned CNTs, exit the CNT growth reactor.

In some embodiments, CNT-infused carbon fiber materials can pass throughyet another treatment process that, in some embodiments is a plasmaprocess used to functionalize the CNTs. Additional functionalization ofCNTs can be used to promote their adhesion to particular resins. Thus,in some embodiments, the present invention provides CNT-infused carbonfiber materials having functionalized CNTs.

As part of the continuous processing of spoolable carbon fibermaterials, the a CNT-infused carbon fiber material can further passthrough a sizing dip bath to apply any additional sizing agents whichcan be beneficial in a final product. Finally if wet winding is desired,the CNT-infused carbon fiber materials can be passed through a resinbath and wound on a mandrel or spool. The resulting carbon fibermaterial/resin combination locks the CNTs on the carbon fiber materialallowing for easier handling and composite fabrication. In someembodiments, CNT infusion is used to provide improved filament winding.Thus, CNTs formed on carbon fibers such as carbon tow, are passedthrough a resin bath to produce resin-impregnated, CNT-infused carbontow. After resin impregnation, the carbon tow can be positioned on thesurface of a rotating mandrel by a delivery head. The tow can then bewound onto the mandrel in a precise geometric pattern in known fashion.

The winding process described above provides pipes, tubes, or otherforms as are characteristically produced via a male mold. But the formsmade from the winding process disclosed herein differ from thoseproduced via conventional filament winding processes. Specifically, inthe process disclosed herein, the forms are made from compositematerials that include CNT-infused tow. Such forms will thereforebenefit from enhanced strength and the like, as provided by theCNT-infused tow.

In some embodiments, a continuous process for infusion of CNTs onspoolable carbon fiber materials can achieve a linespeed between about0.5 ft/min to about 36 ft/min. In this embodiment where the CNT growthchamber is 3 feet long and operating at a 750° C. growth temperature,the process can be run with a linespeed of about 6 ft/min to about 36ft/min to produce, for example, CNTs having a length between about 1micron to about 10 microns. The process can also be run with a linespeedof about 1 ft/min to about 6 ft/min to produce, for example, CNTs havinga length between about 10 microns to about 100 microns. The process canbe run with a linespeed of about 0.5 ft/min to about 1 ft/min toproduce, for example, CNTs having a length between about 100 microns toabout 200 microns. The CNT length is not tied only to linespeed andgrowth temperature, however, the flow rate of both the carbon feedstockand the inert carrier gases can also influence CNT length. For example,a flow rate consisting of less than 1% carbon feedstock in inert gas athigh linespeeds (6 ft/min to 36 ft/min) will result in CNTs having alength between 1 micron to about 5 microns. A flow rate consisting ofmore than 1% carbon feedstock in inert gas at high linespeeds (6 ft/minto 36 ft/min) will result in CNTs having length between 5 microns toabout 10 microns.

In some embodiments, more than one carbon material can be runsimultaneously through the process. For example, multiple tapes tows,filaments, strand and the like can be run through the process inparallel. Thus, any number of pre-fabricated spools of carbon fibermaterial can be run in parallel through the process and re-spooled atthe end of the process. The number of spooled carbon fiber materialsthat can be run in parallel can include one, two, three, four, five,six, up to any number that can be accommodated by the width of theCNT-growth reaction chamber. Moreover, when multiple carbon fibermaterials are run through the process, the number of collection spoolscan be less than the number of spools at the start of the process. Insuch embodiments, carbon strands, tows, or the like can be sent througha further process of combining such carbon fiber materials into higherordered carbon fiber materials such as woven fabrics or the like. Thecontinuous process can also incorporate a post processing chopper thatfacilitates the formation CNT-infused chopped fiber mats, for example.

In some embodiments, processes of the invention allow for synthesizing afirst amount of a first type of carbon nanotube on the carbon fibermaterial, in which the first type of carbon nanotube is selected toalter at least one first property of the carbon fiber material.Subsequently, process of the invention allow for synthesizing a secondamount of a second type of carbon nanotube on the carbon fiber material,in which the second type of carbon nanotube is selected to alter atleast one second property of the carbon fiber material.

In some embodiments, the first amount and second amount of CNTs aredifferent. This can be accompanied by a change in the CNT type or not.Thus, varying the density of CNTs can be used to alter the properties ofthe original carbon fiber material, even if the CNT type remainsunchanged. CNT type can include CNT length and the number of walls, forexample. In some embodiments the first amount and the second amount arethe same. If different properties are desirable in this case along thetwo different stretches of the spoolable material, then the CNT type canbe changed, such as the CNT length. For example, longer CNTs can beuseful in electrical/thermal applications, while shorter CNTs can beuseful in mechanical strengthening applications.

In light of the aforementioned discussion regarding altering theproperties of the carbon fiber materials, the first type of carbonnanotube and the second type of carbon nanotube can be the same, in someembodiments, while the first type of carbon nanotube and the second typeof carbon nanotube can be different, in other embodiments. Likewise, thefirst property and the second property can be the same, in someembodiments. For example, the EMI shielding property can be the propertyof interest addressed by the first amount and type of CNTs and the 2ndamount and type of CNTs, but the degree of change in this property canbe different, as reflected by differing amounts, and/or types of CNTsemployed. Finally, in some embodiments, the first property and thesecond property can be different. Again this may reflect a change in CNTtype. For example the first property can be mechanical strength withshorter CNTs, while the second property can be electrical/thermalproperties with longer CNTs. One skilled in the art will recognize theability to tailor the properties of the carbon fiber material throughthe use of different CNT densities, CNT lengths, and the number of wallsin the CNTs, such as single-walled, double-walled, and multi-walled, forexample.

In some embodiments, processes of the present invention providessynthesizing a first amount of carbon nanotubes on a carbon fibermaterial, such that this first amount allows the carbon nanotube-infusedcarbon fiber material to exhibit a second group of properties thatdiffer from a first group of properties exhibited by the carbon fibermaterial itself. That is, selecting an amount that can alter one or moreproperties of the carbon fiber material, such as tensile strength. Thefirst group of properties and second group of properties can include atleast one of the same properties, thus representing enhancing an alreadyexisting property of the carbon fiber material. In some embodiments, CNTinfusion can impart a second group of properties to the carbonnanotube-infused carbon fiber material that is not included among thefirst group of properties exhibited by the carbon fiber material itself.

In some embodiments, a first amount of carbon nanotubes is selected suchthat the value of at least one property selected from the groupconsisting of tensile strength, Young's Modulus, shear strength, shearmodulus, toughness, compression strength, compression modulus, density,EM wave absorptivity/reflectivity, acoustic transmittance, electricalconductivity, and thermal conductivity of the carbon nanotube-infusedcarbon fiber material differs from the value of the same property of thecarbon fiber material itself.

Tensile strength can include three different measurements: 1) Yieldstrength which evaluates the stress at which material strain changesfrom elastic deformation to plastic deformation, causing the material todeform permanently; 2) Ultimate strength which evaluates the maximumstress a material can withstand when subjected to tension, compressionor shearing; and 3) Breaking strength which evaluates the stresscoordinate on a stress-strain curve at the point of rupture. Compositeshear strength evaluates the stress at which a material fails when aload is applied perpendicular to the fiber direction. Compressionstrength evaluates the stress at which a material fails when acompressive load is applied.

Multiwalled carbon nanotubes, in particular, have the highest tensilestrength of any material yet measured, with a tensile strength of 63 GPahaving been achieved. Moreover, theoretical calculations have indicatedpossible tensile strengths of CNTs of about 300 GPa. Thus, CNT-infusedcarbon fiber materials are expected to have substantially higherultimate strength compared to the parent carbon fiber material. Asdescribed above, the increase in tensile strength will depend on theexact nature of the CNTs used as well as the density and distribution onthe carbon fiber material. CNT-infused carbon fiber materials canexhibit a tow to three times increase in tensile properties, forexample. Exemplary CNT-infused carbon fiber materials can have as highas three times the shear strength as the parent unfunctionalized carbonfiber material and as high as 2.5 times the compression strength.

Young's modulus is a measure of the stiffness of an isotropic elasticmaterial. It is defined as the ratio of the uniaxial stress over theuniaxial strain in the range of stress in which Hooke's Law holds. Thiscan be experimentally determined from the slope of a stress-strain curvecreated during tensile tests conducted on a sample of the material.

Electrical conductivity or specific conductance is a measure of amaterial's ability to conduct an electric current. CNTs with particularstructural parameters such as the degree of twist, which relates to CNTchirality, can be highly conducting, thus exhibiting metallicproperties. A recognized system of nomenclature (M. S. Dresselhaus, etal. Science of Fullerenes and Carbon Nanotubes, Academic Press, SanDiego, Calif. pp. 756-760, (1996)) has been formalized and is recognizedby those skilled in the art with respect to CNT chirality. Thus, forexample, CNTs are distinguished from each other by a double index (n,m)where n and m are integers that describe the cut and wrapping ofhexagonal graphite so that it makes a tube when it is wrapped onto thesurface of a cylinder and the edges are sealed together. When the twoindices are the same, m=n, the resultant tube is said to be of the“arm-chair” (or n,n) type, since when the tube is cut perpendicular tothe CNT axis only the sides of the hexagons are exposed and theirpattern around the periphery of the tube edge resembles the arm and seatof an arm chair repeated n times. Arm-chair CNTs, in particular SWNTs,are metallic, and have extremely high electrical and thermalconductivity. In addition, such SWNTs have-extremely high tensilestrength.

In addition to the degree of twist CNT diameter also effects electricalconductivity. As described above, CNT diameter can be controlled by useof controlled size CNT-forming catalyst nanoparticles. CNTs can also beformed as semi-conducting materials. Conductivity in multi-walled CNTs(MWNTs) can be more complex. Interwall reactions within MWNTs canredistribute current over individual tubes non-uniformly. By contrast,there is no change in current across different parts of metallicsingle-walled nanotubes (SWNTs). Carbon nanotubes also have very highthermal conductivity, comparable to diamond crystal and in-planegraphite sheet.

The CNT-infused carbon fiber materials can benefit from the presence ofCNTs not only in the properties described above, but can also providelighter materials in the process. Thus, such lower density and higherstrength materials translates to greater strength to weight ratio. It isunderstood that modifications which do not substantially affect theactivity of the various embodiments of this invention are also includedwithin the definition of the invention provided herein. Accordingly, thefollowing examples are intended to illustrate but not limit the presentinvention.

EXAMPLE I

This example shows how a carbon fiber material can be infused with CNTsin a continuous process to target thermal and electrical conductivityimprovements.

In this example, the maximum loading of CNTs on fibers is targeted.34-700 12 k carbon fiber tow with a tex value of 800 (Grafil Inc.,Sacramento, Calif.) is implemented as the carbon fiber substrate. Theindividual filaments in this carbon fiber tow have a diameter ofapproximately 7 μm.

FIG. 8 depicts system 800 for producing CNT-infused fiber in accordancewith the illustrative embodiment of the present invention. System 800includes a carbon fiber material payout and tensioner station 805,sizing removal and fiber spreader station 810, plasma treatment station815, barrier coating application station 820, air dry station 825,catalyst application station 830, solvent flash-off station 835,CNT-infusion station 840, fiber bundler station 845, and carbon fibermaterial uptake bobbin 850, interrelated as shown.

Payout and tension station 805 includes payout bobbin 806 and tensioner807. The payout bobbin delivers carbon fiber material 860 to theprocess; the fiber is tensioned via tensioner 807. For this example, thecarbon fiber is processed at a linespeed of 2 ft/min.

Fiber material 860 is delivered to sizing removal and fiber spreaderstation 810 which includes sizing removal heaters 865 and fiber spreader870. At this station, any “sizing” that is on fiber 860 is removed.Typically, removal is accomplished by burning the sizing off of thefiber. Any of a variety of heating means can be used for this purpose,including, for example, an infrared heater, a muffle furnace, and othernon-contact heating processes. Sizing removal can also be accomplishedchemically. The fiber spreader separates the individual elements of thefiber. Various techniques and apparatuses can be used to spread fiber,such as pulling the fiber over and under flat, uniform-diameter bars, orover and under variable-diameter bars, or over bars withradially-expanding grooves and a kneading roller, over a vibratory bar,etc. Spreading the fiber enhances the effectiveness of downstreamoperations, such as plasma application, barrier coating application, andcatalyst application, by exposing more fiber surface area.

Multiple sizing removal heaters 865 can be placed throughout the fiberspreader 870 which allows for gradual, simultaneous desizing andspreading of the fibers. Payout and tension station 805 and sizingremoval and fiber spreader station 810 are routinely used in the fiberindustry; those skilled in the art will be familiar with their designand use.

The temperature and time required for burning off the sizing vary as afunction of (1) the sizing material and (2) the commercialsource/identity of carbon fiber material 860. A conventional sizing on acarbon fiber material can be removed at about 650° C. At thistemperature, it can take as long as 15 minutes to ensure a complete burnoff of the sizing. Increasing the temperature above this burntemperature can reduce burn-off time. Thermogravimetric analysis is usedto determine minimum burn-off temperature for sizing for a particularcommercial product.

Depending on the timing required for sizing removal, sizing removalheaters may not necessarily be included in the CNT-infusion processproper; rather, removal can be performed separately (e.g., in parallel,etc.). In this way, an inventory of sizing-free carbon fiber materialcan be accumulated and spooled for use in a CNT-infused fiber productionline that does not include fiber removal heaters. The sizing-free fiberis then spooled in payout and tension station 805. This production linecan be operated at higher speed than one that includes sizing removal.

Unsized fiber 880 is delivered to plasma treatment station 815. For thisexample, atmospheric plasma treatment is utilized in a ‘downstream’manner from a distance of 1 mm from the spread carbon fiber material.The gaseous feedstock is comprised of 100% helium.

Plasma enhanced fiber 885 is delivered to barrier coating station 820.In this illustrative example, a siloxane-based barrier coating solutionis employed in a dip coating configuration. The solution is ‘AccuglassT-11 Spin-On Glass’ (Honeywell International Inc., Morristown, N.J.)diluted in isopropyl alcohol by a dilution rate of 40 to 1 by volume.The resulting barrier coating thickness on the carbon fiber material isapproximately 40 nm. The barrier coating can be applied at roomtemperature in the ambient environment.

Barrier coated carbon fiber 890 is delivered to air dry station 825 forpartial curing of the nanoscale barrier coating. The air dry stationsends a stream of heated air across the entire carbon fiber spread.Temperatures employed can be in the range of 100° C. to about 500° C.

After air drying, barrier coated carbon fiber 890 is delivered tocatalyst application station 830. In this example, an iron oxide-basedCNT forming catalyst solution is employed in a dip coatingconfiguration. The solution is ‘EFH-1’ (Ferrotec Corporation, Bedford,N.H.) diluted in hexane by a dilution rate of 200 to 1 by volume. Amonolayer of catalyst coating is achieved on the carbon fiber material.‘EFH-1’ prior to dilution has a nanoparticle concentration ranging from3-15% by volume. The iron oxide nanoparticles are of composition Fe₂O₃and Fe₃O₄ and are approximately 8 nm in diameter.

Catalyst-laden carbon fiber material 895 is delivered to solventflash-off station 835. The solvent flash-off station sends a stream ofair across the entire carbon fiber spread. In this example, roomtemperature air can be employed in order to flash-off all hexane left onthe catalyst-laden carbon fiber material.

After solvent flash-off, catalyst-laden fiber 895 is finally advanced toCNT-infusion station 840. In this example, a rectangular reactor with a12 inch growth zone is used to employ CVD growth at atmosphericpressure. 98.0% of the total gas flow is inert gas (Nitrogen) and theother 2.0% is the carbon feedstock (acetylene). The growth zone is heldat 750° C. For the rectangular reactor mentioned above, 750° C. is arelatively high growth temperature, which allows for the highest growthrates possible.

After CNT-infusion, CNT-infused fiber 897 is re-bundled at fiber bundlerstation 845. This operation recombines the individual strands of thefiber, effectively reversing the spreading operation that was conductedat station 810.

The bundled, CNT-infused fiber 897 is wound about uptake fiber bobbin850 for storage. CNT-infused fiber 897 is loaded with CNTs approximately50 μm in length and is then ready for use in composite materials withenhanced thermal and electrical conductivity.

It is noteworthy that some of the operations described above can beconducted under inert atmosphere or vacuum for environmental isolation.For example, if sizing is being burned off of a carbon fiber material,the fiber can be environmentally isolated to contain off-gassing andprevent damage from moisture. For convenience, in system 800,environmental isolation is provided for all operations, with theexception of carbon fiber material payout and tensioning, at thebeginning of the production line, and fiber uptake, at the end of theproduction line.

EXAMPLE II

This example shows how carbon fiber material can be infused with CNTs ina continuous process to target improvements in mechanical properties,especially interfacial characteristics such as shear strength. In thiscase, loading of shorter CNTs on fibers is targeted. In this example,34-700 12 k unsized carbon fiber tow with a tex value of 793 (GrafilInc., Sacramento, Calif.) is implemented as the carbon fiber substrate.The individual filaments in this carbon fiber tow have a diameter ofapproximately 7 μm.

FIG. 9 depicts system 900 for producing CNT-infused fiber in accordancewith the illustrative embodiment of the present invention, and involvesmany of the same stations and processes described in system 800. System900 includes a carbon fiber material payout and tensioner station 902,fiber spreader station 908, plasma treatment station 910, catalystapplication station 912, solvent flash-off station 914, a secondcatalyst application station 916, a second solvent flash-off station918, barrier coating application station 920, air dry station 922, asecond barrier coating application station 924, a second air dry station926, CNT-infusion station 928, fiber bundler station 930, and carbonfiber material uptake bobbin 932, interrelated as shown.

Payout and tension station 902 includes payout bobbin 904 and tensioner906. The payout bobbin delivers carbon fiber material 901 to theprocess; the fiber is tensioned via tensioner 906. For this example, thecarbon fiber is processed at a linespeed of 2 ft/min.

Fiber material 901 is delivered to fiber spreader station 908. As thisfiber is manufactured without sizing, a sizing removal process is notincorporated as part of fiber spreader station 908. The fiber spreaderseparates the individual elements of the fiber in a similar manner asdescribed in fiber spreader 870.

Fiber material 901 is delivered to plasma treatment station 910. Forthis example, atmospheric plasma treatment is utilized in a ‘downstream’manner from a distance of 12 mm from the spread carbon fiber material.The gaseous feedstock is comprised of oxygen in the amount of 1.1% ofthe total inert gas flow (helium). Controlling the oxygen content on thesurface of carbon fiber material is an effective way of enhancing theadherence of subsequent coatings, and is therefore desirable forenhancing mechanical properties of a carbon fiber composite.

Plasma enhanced fiber 911 is delivered to catalyst application station912. In this example, an iron oxide based CNT forming catalyst solutionis employed in a dip coating configuration. The solution is ‘EFH-1’(Ferrotec Corporation, Bedford, N.H.) diluted in hexane by a dilutionrate of 200 to 1 by volume. A monolayer of catalyst coating is achievedon the carbon fiber material. ‘EFH-1’ prior to dilution has ananoparticle concentration ranging from 3-15% by volume. The iron oxidenanoparticles are of composition Fe₂O₃ and Fe₃O₄ and are approximately 8nm in diameter.

Catalyst-laden carbon fiber material 913 is delivered to solventflash-off station 914. The solvent flash-off station sends a stream ofair across the entire carbon fiber spread. In this example, roomtemperature air can be employed in order to flash-off all hexane left onthe catalyst-laden carbon fiber material.

After solvent flash-off, catalyst laden fiber 913 is delivered tocatalyst application station 916, which is identical to catalystapplication station 912. The solution is ‘EFH-1’ diluted in hexane by adilution rate of 800 to 1 by volume. For this example, a configurationwhich includes multiple catalyst application stations is utilized tooptimize the coverage of the catalyst on the plasma enhanced fiber 911.

Catalyst-laden carbon fiber material 917 is delivered to solventflash-off station 918, which is identical to solvent flash-off station914.

After solvent flash-off, catalyst-laden carbon fiber material 917 isdelivered to barrier coating application station 920. In this example, asiloxane-based barrier coating solution is employed in a dip coatingconfiguration. The solution is ‘Accuglass T-11 Spin-On Glass’ (HoneywellInternational Inc., Morristown, N.J.) diluted in isopropyl alcohol by adilution rate of 40 to 1 by volume. The resulting barrier coatingthickness on the carbon fiber material is approximately 40 nm. Thebarrier coating can be applied at room temperature in the ambientenvironment.

Barrier coated carbon fiber 921 is delivered to air dry station 922 forpartial curing of the barrier coating. The air dry station sends astream of heated air across the entire carbon fiber spread. Temperaturesemployed can be in the range of 100° C. to about 500° C.

After air drying, barrier coated carbon fiber 921 is delivered tobarrier coating application station 924, which is identical to barriercoating application station 820. The solution is ‘Accuglass T-11 Spin-OnGlass’ diluted in isopropyl alcohol by a dilution rate of 120 to 1 byvolume. For this example, a configuration which includes multiplebarrier coating application stations is utilized to optimize thecoverage of the barrier coating on the catalyst-laden fiber 917.

Barrier coated carbon fiber 925 is delivered to air dry station 926 forpartial curing of the barrier coating, and is identical to air drystation 922.

After air drying, barrier coated carbon fiber 925 is finally advanced toCNT-infusion station 928. In this example, a rectangular reactor with a12 inch growth zone is used to employ CVD growth at atmosphericpressure. 97.75% of the total gas flow is inert gas (Nitrogen) and theother 2.25% is the carbon feedstock (acetylene). The growth zone is heldat 650° C. For the rectangular reactor mentioned above, 650° C. is arelatively low growth temperature, which allows for the control ofshorter CNT growth.

After CNT-infusion, CNT-infused fiber 929 is re-bundled at fiber bundler930. This operation recombines the individual strands of the fiber,effectively reversing the spreading operation that was conducted atstation 908.

The bundled, CNT-infused fiber 931 is wound about uptake fiber bobbin932 for storage. CNT-infused fiber 929 is loaded with CNTs approximately5 μm in length and is then ready for use in composite materials withenhanced mechanical properties.

In this example, the carbon fiber material passes through catalystapplication stations 912 and 916 prior to barrier coating applicationstations 920 and 924. This ordering of coatings is in the ‘reverse’order as illustrated in Example I, which can improve anchoring of theCNTs to the carbon fiber substrate. During the CNT growth process, thebarrier coating layer is lifted off of the substrate by the CNTs, whichallows for more direct contact with the carbon fiber material (viacatalyst NP interface). Because increases in mechanical properties, andnot thermal/electrical properties, are being targeted, a ‘reverse’ ordercoating configuration is desirable.

It is noteworthy that some of the operations described above can beconducted under inert atmosphere or vacuum for environmental isolation.For convenience, in system 900, environmental isolation is provided forall operations, with the exception of carbon fiber material payout andtensioning, at the beginning of the production line, and fiber uptake,at the end of the production line.

EXAMPLE III

This example shows how carbon fiber material can be infused with CNTs ina continuous process to target improvements in mechanical properties,especially interfacial characteristics such as interlaminar shear.

In this example, loading of shorter CNTs on fibers is targeted. In thisexample, 34-700 12 k unsized carbon fiber tow with a tex value of 793(Grafil Inc., Sacramento, Calif.) is implemented as the carbon fibersubstrate. The individual filaments in this carbon fiber tow have adiameter of approximately 7 μm.

FIG. 10 depicts system 1000 for producing CNT-infused fiber inaccordance with the illustrative embodiment of the present invention,and involves many of the same stations and processes described in system800. System 1000 includes a carbon fiber material payout and tensionerstation 1002, fiber spreader station 1008, plasma treatment station1010, coating application station 1012, air dry station 1014, a secondcoating application station 1016, a second air dry station 1018,CNT-infusion station 1020, fiber bundler station 1022, and carbon fibermaterial uptake bobbin 1024, interrelated as shown.

Payout and tension station 1002 includes payout bobbin 1004 andtensioner 1006. The payout bobbin delivers carbon fiber material 1001 tothe process; the fiber is tensioned via tensioner 1006. For thisexample, the carbon fiber is processed at a linespeed of 5 ft/min.

Fiber material 1001 is delivered to fiber spreader station 1008. As thisfiber is manufactured without sizing, a sizing removal process is notincorporated as part of fiber spreader station 1008. The fiber spreaderseparates the individual elements of the fiber in a similar manner asdescribed in fiber spreader 870.

Fiber material 1001 is delivered to plasma treatment station 1010. Forthis example, atmospheric plasma treatment is utilized in a ‘downstream’manner from a distance of 12 mm from the spread carbon fiber material.The gaseous feedstock is comprised of oxygen in the amount of 1.1% ofthe total inert gas flow (helium). Controlling the oxygen content on thesurface of carbon fiber material is an effective way of enhancing theadherence of subsequent coatings, and is therefore desirable forenhancing mechanical properties of a carbon fiber composite.

Plasma enhanced fiber 1011 is delivered to coating application station1012. In this example, an iron oxide based catalyst and a barriercoating material is combined into a single ‘hybrid’ solution and isemployed in a dip coating configuration. The ‘hybrid’ solution is1-part-by-volume ‘EFH-1’, 5-parts ‘Accuglass T-11 Spin-On Glass’,24-parts hexane, 24-parts isopropyl alcohol, and 146-partstetrahydrofuran. The benefit of employing such a ‘hybrid’ coating isthat it marginalizes the effect of fiber degradation at hightemperatures. Without being bound by theory, degradation to carbon fibermaterials is intensified by the sintering of catalyst NPs at hightemperatures (the same temperatures vital to the growth of CNTs). Byencapsulating each catalyst NP with its own barrier coating, it ispossible to control this effect. Because increases in mechanicalproperties, and not thermal/electrical properties, is being targeted, itis desirable to maintain the integrity of the carbon fiberbase-material, therefore a ‘hybrid’ coating can be employed.

Catalyst-laden and barrier coated carbon fiber material 1013 isdelivered to air dry station 1014 for partial curing of the barriercoating. The air dry station sends a stream of heated air across theentire carbon fiber spread. Temperatures employed can be in the range of100° C. to about 500° C.

After air drying, the catalyst and barrier coating-laden carbon fiber1013 is delivered to coating application station 1016, which isidentical to coating application station 1012. The same ‘hybrid’solution is used (1-part-by-volume ‘EFH-1’, 5-parts ‘Accuglass T-11Spin-On Glass’, 24-parts hexane, 24-parts isopropyl alcohol, and146-parts tetrahydrofuran). For this example, a configuration whichincludes multiple coating application stations is utilized to optimizedthe coverage of the ‘hybrid’ coating on the plasma enhanced fiber 1011.

Catalyst and barrier coating-laden carbon fiber 1017 is delivered to airdry station 1018 for partial curing of the barrier coating, and isidentical to air dry station 1014.

After air drying, catalyst and barrier coating-laden carbon fiber 1017is finally advanced to CNT-infusion station 1020. In this example, arectangular reactor with a 12 inch growth zone is used to employ CVDgrowth at atmospheric pressure. 98.7% of the total gas flow is inert gas(Nitrogen) and the other 1.3% is the carbon feedstock (acetylene). Thegrowth zone is held at 675° C. For the rectangular reactor mentionedabove, 675° C. is a relatively low growth temperature, which allows forthe control of shorter CNT growth.

After CNT-infusion, CNT-infused fiber 1021 is re-bundled at fiberbundler 1022. This operation recombines the individual strands of thefiber, effectively reversing the spreading operation that was conductedat station 1008.

The bundled, CNT-infused fiber 1021 is wound about uptake fiber bobbin1024 for storage. CNT-infused fiber 1021 is loaded with CNTsapproximately 2 μm in length and is then ready for use in compositematerials with enhanced mechanical properties.

It is noteworthy that some of the operations described above can beconducted under inert atmosphere or vacuum for environmental isolation.For convenience, in system 1000, environmental isolation is provided forall operations, with the exception of carbon fiber material payout andtensioning, at the beginning of the production line, and fiber uptake,at the end of the production line.

It is to be understood that the above-described embodiments are merelyillustrative of the present invention and that many variations of theabove-described embodiments can be devised by those skilled in the artwithout departing from the scope of the invention. For example, in thisSpecification, numerous specific details are provided in order toprovide a thorough description and understanding of the illustrativeembodiments of the present invention. Those skilled in the art willrecognize, however, that the invention can be practiced without one ormore of those details, or with other processes, materials, components,etc.

Furthermore, in some instances, well-known structures, materials, oroperations are not shown or described in detail to avoid obscuringaspects of the illustrative embodiments. It is understood that thevarious embodiments shown in the Figures are illustrative, and are notnecessarily drawn to scale. Reference throughout the specification to“one embodiment” or “an embodiment” or “some embodiments” means that aparticular feature, structure, material, or characteristic described inconnection with the embodiment(s) is included in at least one embodimentof the present invention, but not necessarily all embodiments.Consequently, the appearances of the phrase “in one embodiment,” “in anembodiment,” or “in some embodiments” in various places throughout theSpecification are not necessarily all referring to the same embodiment.Furthermore, the particular features, structures, materials, orcharacteristics can be combined in any suitable manner in one or moreembodiments. It is therefore intended that such variations be includedwithin the scope of the following claims and their equivalents.

What is claimed is the following:
 1. A continuous CNT infusion processcomprising: functionalizing a carbon fiber material to form afunctionalized carbon fiber material; disposing a barrier coating onsaid functionalized carbon fiber material; wherein the barrier coatingis selected from the group consisting of an alkoxysilane, amethylsiloxane, an alumoxane, alumina nanoparticles, spin on glass andglass nanoparticles; disposing a carbon nanotube (CNT)-forming catalyston said functionalized carbon fiber material; and synthesizing carbonnanotubes on said functionalized carbon fiber material, thereby forminga carbon nanotube-infused carbon fiber material; wherein thefunctionalized carbon fiber material is moving while disposing thebarrier coating and the CNT-forming catalyst, and while synthesizing thecarbon nanotubes.
 2. The process of claim 1, wherein said continuous CNTinfusion process has a material residence time in a CNT growth chamberof between about 5 to about 300 seconds.
 3. The process of claim 2,wherein a material residence time in the CNT growth chamber of about 5to about 30 seconds produces CNTs having a length between about 1 micronto about 10 microns.
 4. The process of claim 2, wherein a materialresidence time in the CNT growth chamber of about 30 to about 180seconds produces CNTs having a length between about 10 microns to about100 microns.
 5. The process of claim 2, wherein a material residencetime in the CNT growth chamber of about 180 to about 300 secondsproduces CNTs having a length between about 100 microns to about 500microns.
 6. The process of claim 1, wherein more than one carbon fibermaterial is run simultaneously through the process.
 7. The process ofclaim 1, wherein said functionalizing said carbon fiber materialcomprises exposing said carbon fiber material to a plasma.
 8. Theprocess of claim 1 further comprising: removing a sizing material fromsaid carbon fiber material.
 9. The process of claim 1, wherein saidCNT-forming catalyst is an iron-based nanoparticle catalyst.
 10. Theprocess of claim 1, wherein disposing said CNT-forming catalyst on saidfunctionalized carbon fiber material comprises spraying, dip coating, orgas phase deposition onto said functionalized carbon fiber material. 11.The process of claim 1, wherein disposing said barrier coating issimultaneous with disposing said CNT-forming catalyst on saidfunctionalized carbon fiber material.
 12. The process of claim 1,wherein said barrier coating is conformally disposed on saidfunctionalized carbon fiber material prior to disposing said CNT-formingcatalyst on said functionalized carbon fiber material.
 13. A continuousCNT infusion process comprising: functionalizing a carbon fiber materialto form a functionalized carbon fiber material; disposing a barriercoating on said functionalized carbon fiber material; partially curingsaid barrier coating and then disposing a carbon nanotube (CNT)-formingcatalyst on said functionalized carbon fiber material; wherein saidbarrier coating is conformally disposed on said functionalized carbonfiber material prior to disposing said CNT-forming catalyst on saidfunctionalized carbon fiber material; and synthesizing carbon nanotubeson said functionalized carbon fiber material, thereby forming a carbonnanotube-infused carbon fiber material; wherein the functionalizedcarbon fiber material is moving while disposing the barrier coating andthe CNT-forming catalyst, and while synthesizing the carbon nanotubes.14. The process of claim 13 further comprising: curing the barriercoating after disposing said CNT-forming catalyst on said functionalizedcarbon fiber material.
 15. A continuous CNT infusion process comprising:functionalizing a carbon fiber material to form a functionalized carbonfiber material; disposing a barrier coating on said functionalizedcarbon fiber material; fully curing said barrier coating to form a fullycured barrier coating; exposing said fully cured barrier coating to aplasma and then disposing a carbon nanotube (CNT)-forming catalyst onsaid functionalized carbon fiber material; and synthesizing carbonnanotubes on said functionalized carbon fiber material, thereby forminga carbon nanotube-infused carbon fiber material; wherein thefunctionalized carbon fiber material is moving while disposing thebarrier coating and the CNT-forming catalyst, and while synthesizing thecarbon nanotubes.
 16. The process of claim 1, wherein synthesizingcarbon nanotubes comprises CVD growth.
 17. The process of claim 1further comprising: after synthesizing the carbon nanotubes, applyingsizing to said carbon nanotube-infused carbon fiber material.
 18. Theprocess of claim 1, further comprising: after synthesizing the carbonnanotubes, applying a matrix material to said carbon nanotube-infusedcarbon fiber material.
 19. The process of claim 1, wherein the barriercoating is deposited from a liquid onto the functionalized carbon fibermaterial.